In the transportation of products, in particular hydrocarbonaceous products, by transport vessels, several problems are often encountered. These problems include product biodegradation, product oxidation, corrosion, transfer of invasive species, and fires and explosions. During transportation, the hydrocarbonaceous products may be subject to biodegradation by microorganisms. In particular, hydrocarbonaceous products are frequently exposed to a layer of water when stored in large storage vessels fuel tanks of aircraft and holds of tankers. In these large storage vessels, water invariably forms due to condensation or it is initially present in the stored hydrocarbonaceous product and slowly separates therefrom. This water gradually forms a layer in the bottom of the storage vessels. The water layer forms an interface with the hydrocarbonaceous product, and becomes a breeding ground for a wide variety of microorganisms. These microorganisms utilize the hydrocarbonaceous product as a nutrient and can multiply.
Eventually the microorganisms can consume a portion of the hydrocarbonaceous product. The extent to which the microorganisms consume the product is known as the extent of biodegradation, or the biodegradability of the product.
The microorganisms or microbes will grow mostly in the water phase, but when the hydrocarbonaceous product is disturbed during pumping or mixing, the microbes can be dispersed into the hydrocarbonaceous product and cause contamination. When present in the hydrocarbonaceous product, microbial growth can present a problem for several reasons. For example, hydrocarbonaceous products may become contaminated with microbes during storage or shipment and as a result of the microbes, become hazy or cloudy. The growing microorganisms may form sludge in the contaminated hydrocarbonaceous product. When contaminated hydrocarbonaceous products are used in an engine or equipment, the microbes and/or the sludge may decrease the efficiency of the engine or equipment or prevent it from functioning altogether, for example, by plugging filters. In addition, growth of microorganisms, in particular anaerobic sulfate reducing bacteria, in hydrocarbonaceous products during storage or transport may create corrosive sulfur-containing acids and damage the vessels in which the products are contained. This corrosion damage may lead to the need for eventual replacement of these large, expensive vessels.
In addition to biodegradation by microorganisms, there is also the potential that microorganisms and higher life forms may be transported to environments previously uninhabited by these organisms. Transport of water on marine tankers, in particular ballast water, contaminated with microbes creates a dispersal mechanism for human pathogens, waterborne diseases of plants and animals, and foreign organisms into the environment. Ballast water is a breeding ground for organisms and is implicated in introducing foreign organisms into the environment. Ballast water is used on ships in order to maintain appropriate ship draft, trim, stability, immersion, and hull integrity. Ballast water may be taken on in special ballast tanks or may be taken on in the cargo tanks of a ship. Ships travel with ballast water when carrying no cargo or light cargo and travel with little or no ballast on board when carrying a maximum cargo. The quantity of ballast water used in a ship can be quite large. For example, a 300,000 metric ton petroleum tanker has a ballast water capacity of 100,000 metric tons. Typically, the ratio of ballast water to ship capacity is 1:3 to 1:2.
Ballast water is normally taken on in one coastal region and is discharged in another coastal region. For example, ships take on ballast water from one port, travel to a second port, and discharge a large amount of ballast water in order to take on cargo at the second port. The discharge of ballast water has led to the introduction of non-native life forms in many areas, as the life forms from one coastal region to another vary. Even ships reporting no ballast on board may act as vectors for non-native life forms because the ballast tanks of such ships contain an unpumpable amount of residual ballast water.
For example, infectious bacteria such as cholera have been found in ballast water from marine tankers (“Global Spread of Microorganisms By Ships,” Brief Communications Nov. 2, 2000 issue of Nature). These infectious organisms can create both a human health problem, and a health problem to native species in the receiving country. Water can also be the vehicle for the introduction of foreign higher life forms into the receiving countries' environment. By this route, Zebra clams are believed to have been introduced into the San Francisco Bay region.
One proposed method of limiting the introduction of foreign organisms into marine environments is for ships to discharge their ballast water in the open ocean prior to entering port without taking on new ballast water. However, as mentioned above, ballast water is typically needed to maintain essential operating conditions of a ship, and it may be dangerous to discharge ballast water before reaching a port.
Another method for limiting the introduction of life forms in ballast water is to exchange near-coast ballast water for mid-ocean ballast water. Presumably, the life forms taken on board the ship in a near-coast environment are flushed into the mid-ocean. It is important that the ship maintain stability, draft, and other operating parameters during this exchange, and only ships that are designed for this practice can safely exchange ballast water. Ships may be retrofitted to permit this exchange, but such retrofitting is very expensive. Currently only a small proportion of the world's cargo fleet is capable of ballast water exchange. Ballast water exchange may be completed by (1) emptying the ballast tanks and refilling them one at a time, or (2) pumping three volumes of ocean water into the tanks to flush them. Neither approach is completely effective in eliminating foreign life forms. The first method leaves a heal in the tank which can harbor life forms, and the second method allows life forms to be retained during the mixing. The effectiveness of ballast water exchange has been estimated at 90% and usually takes about 2 days to safely complete. Because the above methods are not as effective or efficient as ideally desired, other methods of controlling the transfer of life forms need to be proposed.
During transportation, the hydrocarbonaceous products may be subject to oxidation and the ballast and cargo tanks on the ship may be subject to corrosion. During transportation, the hydrocarbonaceous product can oxidize. Although environmentally friendly, Fischer Tropsch products can oxidize relatively rapidly when exposed to air. The rapid oxidation may be due to a lack natural anti-oxidants, such as sulfur compounds. Further, some of the products produced by the Fischer Tropsch process may be waxy, and these products are frequently are shipped at elevated temperature. Shipping at elevated temperatures increases the tendency of Fischer Tropsch products to oxidize.
Corrosion on ships has been linked to several major disasters. Most prominent was the 1999 sinking of the oil tanker Erika which spilled millions of gallons of fuel oil on the coast of France. The French government found that corrosion was one of the strong contributing factors that lead to the sinking of this ship. Other ship losses attributed in part to corrosion include the Nakhodka in 1997 and the Castor in 2000. One method to prevent corrosion is to paint the metal surfaces of ships or coat them with a corrosion-resistant substance. However, it is very difficult to coat all surfaces, and any uncoated surface can lead to problems.
During transportation of hydrocarbonaceous products on marine vessels, there is the potential of fires and explosions on the transport vessels. When transporting methanol, from, for example a methanol synthesis process, methanol is quite flammable and precautions to prevent fires and explosions must be taken when shipping it. Regulations that minimize the chances of the fires and explosions on transport vessels are covered in the Safety Of Life At Sea (SOLAS) international treaty. The 1978 SOLAS Protocol was adopted at the International Conference on Tanker Safety and Pollution Prevention, which was convened in response to a spate of tanker accidents in 1976–1977. As a result, an inert gas system became mandatory for existing petroleum carriers of 70,000 deadweight tons (dwt) and above by May 1, 1983, and for ships of 20,000–70,000 dwt by May 1, 1985.
Requirements for the removal of oxygen, or “inerting the tank”, from the cargo space of volatile petroleum products are recognized in regulations such as the United States Coast Guard regulations. The regulations require that the oxygen content of the gas phase in contact with the petroleum product be less than 8 volume %.
A typical method to inert petroleum cargo spaces is to take exhaust from the steam boilers or from diesel engines used to pump petroleum onto the vessel, scrub it with sea water, and use the scrubbed gas to inert the tanks. The exhaust gas contains carbon dioxide, nitrogen, and low levels of oxygen, sulfur oxides, and nitrogen oxides. Oxygen content of the scrubbed gas is monitored to ensure that it is below 5 volume %, providing assurance that the gas in contact with the petroleum product will be less than 8 volume % oxygen. Scrubbed gas can also be used to inert the ballast tanks. Inerting the cargo tanks is most important when petroleum is being unloaded as some gas must be introduced into the tank to displace the petroleum. During loading, the tank should have been previously inerted, and the petroleum displaces this gas. The steam boilers and diesel engines used to pump petroleum off the vessel are typically located on-board the vessel.
While scrubbed gas is an inexpensive source of inert gas, the scrubbing system may not be completely effective in removing carbon dioxide, nitrogen oxides, and sulfur oxides. Carbon dioxide in the scrubbed gas can lead to corrosion of tanks. Traces of nitrogen oxides, themselves oxidants, can oxidize Fischer-Tropsch products. Finally, traces of sulfur oxides can be incorporated in Fischer-Tropsch products, thereby increasing their sulfur content and reducing their value as low sulfur fuels. Better sources of inexpensive inert gas are desired. What is needed is an efficient and inexpensive way to address the problems of product biodegradation, product oxidation, corrosion, transfer of invasive species, and fires and explosions on transport vessels.